1. Field of the Invention
The present invention relates mainly to a liquid crystal composition suitable for use in an active matrix (AM) element, and an AM element including the composition.
2. Related Art
On a liquid crystal display element, classification based on an operating mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment and so forth. Classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). PM is further classified into static, multiplex and so forth, and AM is classified into a thin film transistor (TFT), a metal insular metal (MIM) and so forth. TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. Classification based on a light source is a reflection type utilizing a natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both the natural light and the backlight.
These elements include a liquid crystal composition having suitable characteristics. General characteristics of the composition should be improved to obtain an AM element having good general characteristics. Table 1 below summarizes a relationship between the two general characteristics. The general characteristics of the composition will be explained further based on a commercially available AM element. A temperature range of a nematic phase relates to the temperature range in which the element can be used. A desirable range at a higher limit temperature of the nematic phase is 70° C. or more and a desirable range at a lower limit temperature is −20° C. or less. The viscosity of the composition relates to the response time of the element. A short response time is desirable for displaying a moving image. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General characteristics of a liquid crystalcomposition and an AM element.General characteristicsGeneral characteristicsNo.of a compositionof an AM element1Temperature range of aUsable temperature rangenematic phase is wideis wide2Viscosity is small1)Response time is short3Optical anisotropy isContrast ratio is largesuitable4Dielectric anisotropy isDriving voltage is lowpositively or negativelyand electric powerlargeconsumption is small5Specific resistance isVoltage holding ratio islargesmall and a contrastratio is large1)Time for injecting a composition into a liquid crystal cell can be shortened. 
The optical anisotropy of the composition relates to the contrast ratio of the element. A VA element makes use of electrically controlled birefringence. Then, a product (Δn·d) of the optical anisotropy (Δn) of a composition and the cell gap (d) of the element is designed to a fixed value in order to maximize a contrast ratio in the AV element. One example of this value is 0.275 μm. The cell gap (d) is usually 3 to 6 μm, and therefore an optical anisotropy in the composition falls in a range of 0.05 to 0.12. A large dielectric anisotropy of the composition contributes to a small driving voltage of the element. Accordingly, a large dielectric anisotropy is preferred. On the other hand, a composition having a negative dielectric anisotropy is used in an AV element of a VA mode. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the element. Accordingly, a composition having a large specific resistance in the initial stage is desirable. A composition having a large specific resistance even after it has been used for a long time is desirable.
A composition having a negative dielectric anisotropy contains a compound having a negative dielectric anisotropy. Such compound has a polar group in a minor axis direction of a compound molecule and is described in the following patent documents. JP H03-027340 A/1991 (U.S. Pat. No. 5,032,312), JP H03-066632 A/1991 (EP 0 410 233), and JP H04-330019 A/1992 (U.S. Pat. No. 5,204,018).